1. Field
The present invention relates to magnetoresistive heads. In particular, the present invention relates to setting an operating bias current for a magnetoresistive head by computing a target operating voltage.
2. Description of the Related Art
Magnetoresistive (MR) heads are typically employed in data storage devices, such as magnetic tape drives and disk drives, for transducing the magnetic transitions recorded on a magnetic medium into a read signal that is demodulated by a read channel. A MR head comprises an MR element having a resistance that varies in response to the magnetic field emanating from the recording medium. The read signal may be generated by applying a constant bias voltage to the MR element and measuring the change in current flowing through the MR element as the resistance varies. Alternatively, the read signal may be generated by applying a constant bias current to the MR element and measuring the change in voltage across the MR element as the resistance varies.
Increasing the bias current of the MR element typically increases the quality of the read signal (increases signal-to-noise); however, setting the bias current too high reduces the lifetime of the MR element. For example, setting the bias current too high can reduce the lifetime of a giant MR element (GMR) due to self-heating and concomitant high current density, and it can reduce the lifetime of a tunneling MR element (TMR) due to dielectric breakdown. The lifetime of the MR element is also typically affected by other operating characteristics, such as the ambient temperature. The prior art has suggested to characterize the MR element using various lifetime testing procedures which applies stresses in order to accelerate the time to failure (e.g., increasing the bias current). The lifetime information is then used to select a nominal bias current setting (taking into account tolerance) for a family of MR elements employed in the field, such as in a family of disk drives. The prior art has also suggested to correlate the bias current settings with MR resistance and ambient temperature during the lifetime testing, and to use this information to adjust the bias current setting while in the field.
Certain MR elements, such as tunneling MR elements, exhibit a negative voltage coefficient of resistance resulting in a non-linear voltage drop at higher current densities as illustrated in FIG. 1. The negative voltage coefficient of resistance makes it very difficult to determine an accurate bias current setting directly as a function of resistance and ambient temperature. An alternative is to generate a multi-dimensional lookup table having parameters such as resistance and temperature as input, and the corresponding bias current setting as the output. However, the size of this table becomes unwieldy, particular as the number of input parameters increases beyond just resistance and ambient temperature.
There is, therefore, a need for an efficient technique to set an operating bias current for an MR element.